Patrick was not the first Christian missionary to Ireland, but he seems to have been the one who made the most impact. Mythology credits him with banishing snakes from the island of Ireland, though others suggest that for climatic reasons Ireland never actually had snakes; one suggestion is that "snakes" referred to the symbolism of the pagan priests of that time and place, possibly shown by their tattoos or that it could have referred to Pelagianism, symbolized as an Old-Testamental "serpent".

Legend also credits Patrick with teaching the Irish about the concept of the Trinity by showing people the shamrock, a three-leaved clover, using it to highlight the Christiandogma of 'three divine persons in the one god' (as opposed to the Arian heresy that was popular in Patrick's time).

It is unknown on what date he was born and died but it is believed that March 17 was either his birth or death date, and it is the date popularly associated with him as his Feast Day (known as St. Patrick's Day).